As a thin-film type electroluminescent (EL) element, those which comprise an inorganic material of the II-VI-group compound semiconductor such as ZnS, CaS or SrS doped with a light-emitting center of Mn or a rare earth element (Eu, Ce, Tb or Sm) have conventionally been popular. However, EL elements prepared from the inorganic material involve the following problems:    1) that they require countercurrent driving (50-1000 Hz);    2) that they require a high driving volt (1-200V);    3) that it is difficult to realize a full-color display (particularly blue color being difficult) by using them; and    4) that they require expensive peripheral equipment-driving circuits.
However, in recent years, development of EL elements using an organic thin film has been started in order to solve the above-mentioned problems. In particular, in order to enhance luminous efficiency, optimization has been conducted as to the kind of electrode for the purpose of improving efficiency of carrier injection from the electrode and, by the development of organic electroluminescent elements wherein a hole transport layer comprising an aromatic diamine and a light-emitting layer comprising an aluminum complex of 8-hydroxyquinoline are provided (see, non-patent document 1: Appl. Phys. Lett., 51, 913, 1987), luminous efficiency has been much more improved in comparison with the conventional EL elements using single crystal of, for example, anthracene. It has also been conducted to use an aluminum complex of 8-hydroxyquinoline as a host material and dope it with a fluorescent dye for laser such as coumarin (see, non-patent document 2: J. Appl. Phys., 65, 3610, 1989) to thereby improve luminous efficiency and conduct conversion of wavelength of emitted light. Thus, practically employable properties have approximately been obtained.
In addition to the electroluminescent elements using the low molecular materials as described above, electroluminescent elements using a high molecular material such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] or poly(3-alkylthiophene) for light-emitting layers and elements wherein a low molecular light-emitting material and an electron transfer element are mixed with a polymer material such as polyvinylcarbazole have been developed.
As an attempt to raise luminous efficiency of the element, it has also been examined to use phosphorescence instead of fluorescence. In comparison with the conventional elements using fluorescence (singlet state), elements using phosphorescence, that is, utilizing light emitted from triplet excitation state are expected to show about 3 times more improved efficiency. For this purpose, it was investigated to form a light-emitting layer comprising a coumarin derivative or a benzophenone derivative (see, non-patent document 3: 51th Oyo Butsurigakukai Rengo Koenkai, 28a-PB-7, 1990). However, there was obtained an extremely low lumninance. Thereafter, as an attempt to utilize triplet state, use of a europium complex has been investigated, but this attempt did not lead to realization of high luminous efficiency.
Recently, it has been reported that a red light can be emitted with high efficiency by using a platinum complex (T-1) shown below (non-patent document 4: Nature, 395, 151, 1998). Then, efficiency of emitting a green light has been markedly improved by doping an iridium complex (T-2) shown below into a light-emitting layer (non-patent document 5: App. Phys. Lett., 75, 4, 1999).

In order to apply an organic electroluminescent element to a display element such as a flat panel display, it is necessary to ensure sufficient stability upon driving as well as to improve luminous efficiency of the light-emitting element.
However, the organic electroluminescent element using the phosphorescence-emitting molecule (T-2) described in the foregoing literature shows a practically insufficient driving stability though it shows a high luminous efficiency (see, non-patent document 6: Jpn. J. Appl. Phys., 38, L1502, 1999). Thus, under the present situation, a display element showing a high efficiency is difficult to realize.
As a novel material system, patent document 1 (JP-A-2003-123983) proposes pyridine-based compounds represented by the following compounds as materials for an electron transport layer or a light-emitting layer of an organic electroluminescent element.

However, these compounds have a structure wherein nitrogen atoms in respective pyridine rings can be conjugated to each other, and hence they show a comparatively small oxidation-reduction potential difference.
Generally, in order to prepare organic electroluminescent elements emitting blue fluorescence or green to blue phosphorescence, it is required to use light-emitting dye having an extremely large oxidation-reduction potential difference and, in order to supply and focus charge for the dye with a high efficiency, materials surrounding the dye (a host material in a light-emitting layer and a charge transporting material constituting a layer adjacent to the light-emitting layer) are required to have a larger oxidation-reduction potential difference than that of the dye. Therefore, application of the pyridine-based compounds described in patent document 1 to the blue fluorescence-emitting element or to the phosphorescence-emitting element is considered to be difficult.
Also, since the pyridine-based compounds have hydrogen atoms at the 2-, 4- or 6-position which are Active sites on the pyridine ring, they involve a problem as to electrochemical stability. Thus, for using them as charge transporting materials in electroluminescent elements, they must be more improved.
Further, in the case where the light-emitting element is a metal complex, incorporation of a compound having a unit with a strong coordinating ability such as a bipyridyl group in a light-emitting layer or in a layer adjacent thereto can cause ligand exchange when an electric field is applied for a long time.
Non-patent document 7 (collect. Czech. Chem. Commun. (vol. 57) (1992)) proposed fluorescent materials represented by the following formulae:

The document proposes to use the above-described compounds mainly as fluorescent dyes emitting a blue light, and does not disclose other specific applications.
By the way, in the organic electroluminescent elements having so far been reported, light emission is obtained fundamentally by the combination of a hole transport layer and an electron transport layer. The principle of emission of light is that holes injected from an anode migrate through the hole transport layer and recombine with electrons having injected from a cathode and having migrated through an electron transport layer in the vicinity of interface between the two layers to thereby excite the hole transport layer and/or the electron transport layer. Generally, a light-emitting layer is provided between the hole transport layer and the electron transport layer to thereby improve luminous efficiency.
Further, in some cases, a hole blocking layer is provided in contact with the interface with the light-emitting layer on the cathode side for the purpose of accelerating generation of excitons in the light-emitting layer to thereby enhance efficiency of light emission and purity of the color of emitted light. In particular, in an element wherein a triarylamine-based compound is used in a hole injection/transport layer and an aluminum complex is used in an electron injection/transport layer, mobility of hole tends to exceed mobility of electron, leading to the problem that holes pass through to the cathode side without contributing to emission of light. Particularly with an element wherein oxidation potential of the light-emitting layer is large and an electron transport layer uses commonly employed Alq3 (an aluminum complex of 8-hydroxyquinoline), the hole blocking layer is highly required with a blue color light-emitting element or a phosphorescence-emitting element wherein holes are difficultly confined in the light-emitting layer.
With respect to the hole blocking layer, patent document 2 (JP-A-2-195683) describes, for example, to provide a hole blocking layer having a larger ionization potential than that of the light-emitting layer and, as an example thereof, it proposes to use tris(5,7-dichloroo-8-hydroxyquinolino) aluminum. Also, patent document 3 (JP-A-9-87616) proposes to use silacyclopentadiene. However, these fail to provide sufficient driving stability.
As the causes for the deterioration of driving, there have been pointed out thermal deterioration due to a low glass transition temperature (Tg) of the hole blocking material and an electrochemical factor that the hole blocking material is reduced or oxidized by injection of electrons or holes.
In an element using an iridium complex as a light-emitting dye, which emits phosphorescence with a high efficiency, an aluminum complex type hole blocking material such as Balq (aluminum(III) bis(2-methyl-8-quinolinato 4-phenylphenolate) or SAlq (aluminum(III) bis(2-methyl-8-quinolinato)triphenylsilanolate) is popularly used, succeeding in obtaining a long life to some extent (see, non-patent document 8: Appl. Phys. Lett., vol. 81, p. 162 (2002)).
However, insufficient hole blocking ability of the above-mentioned compounds has caused the problem of insufficient luminous efficiency of the element and the problem of oxidative deterioration of the material for the electron transport layer due to the fact that part of holes pass through the hole blocking material to the electron transport layer.
With the above-mentioned reasons, it has been necessary to realize rapid charge recombination in the light-emitting layer and high luminous efficiency of a dopant and to give the hole blocking material itself an enough durability against electric oxidation and reduction. Thus, further improvements and investigations have been desired with respect to materials for preparing a stable element which emits light with a high efficiency and an element structure for such materials.
[Patent Document 1]
JP-A-2003-123987
[Patent Document 2]
JP-A-2-195683
[Patent Document 3]
JP-A-9-87616
[Non-Patent Document 1]
Appl. Phys. Lett., vol. 51, p. 913, 1987
[Non-Patent Document 2]
J. Appl. Phys., vol. 65, p. 3610, 1989
[Non-Patent Document 3]
The 51th Oyo Butsuri-gakkai Rengo Koennkai, 28a-PB-7, 1990
[Non-Patent Document 4]
Nature, vol. 395, p. 151, 1998
[Non-Patent Document 5]
Appl. Phys. Lett., vol. 75, p. 4, 1999
[Non-Patent Document 6]
Jpn. J. Phys., vol. 38, L1502, 1999
[Non-Patent Document 7]
Collect. Czech. Chem. Commun. (Vol. 57)(1992)
[Non-Patent Document 8]
Appl. Phys. Lett., vol 81, p. 162, 2002